In the formation of a semiconductor wafer, integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate. Interconnect structures are then formed over the integrated circuit devices. Metal bumps are formed on the surface of the semiconductor chip, so that the integrated circuit devices can be accessed.
In a typical metal-bump formation process, an under-bump metallurgy (UBM) layer is first formed to electrically couple to a metal pad. The UBM layer may include a titanium layer, and a copper seed layer over the titanium layer. Metal bumps are then formed on the UBM layer, for example, by plating. The formation process includes forming a mask to cover first portions of the UBM layer, and leave second portions of the UBM layer un-covered. The metal bumps are formed on the second portions of the UBM layer. After the formation of the metal bumps, the mask is removed, and the first portions of UBM layer are removed by wet etching. It was observed that the wet etching caused undercuts to be formed under the metal bumps due to the lateral etching of the titanium layer. As a result, metal bumps may delaminate from the respective chip or wafer, resulting in a low yield in the metal bump formation process.